Looks like the PC repairman didn't learn anything from an older, more
experienced PC assembler.
In the old days you looked for the pin 1 end of the device connector and
pushed the pin 1 of the cable connector into it. Half of them had a twist or
were put on backwards and had no "key" in them.
There was no advise from others, we were cutting new ground and the
standards were not developed yet so we learned the hard way. Somebody has to
do it first.
A PC repairman told me about fixing a machine where the owner had
carved off the IDE plug keying so he could insert it backwards.
There's also an occasional poster on rec.crafts.metalworking who
always wants to know how to do something dangerous that he doesn't
understand in the cheapest possible way. They aren't common, but AFAIK
they are the reason for the warning.
No, I didn't learn the hard way. I took advice that several sources